Introduction to Mobile communications
Why Wireless? Basic Communications System Elements ● Source ● Destination ● Transmission Medium
Source
Network
Transmission Medium
Destination
Transition medium Transmission Medium
Wire
Twisted-pair cable
Coaxial cable
Wireless
Fiber-optic cable
LOS
Non-LOS
What Wireless?
Why Wireless?
Drawbacks of wireless communication
Types of communication systems
TX
TX+RX
TX+RX
RX
TX +RX
TX +RX
Duplex Transmission & Multiple Access
Duplex Transmission
Multiple Access
Multiple-access for Digital Communication Systems • The frequency spectrum must be shared by all the users in the system. • Three method for sharing spectrum: – FDMA • Frequency-division multiple-access.
– TDMA • Time-division multiple-access.
– CDMA • Code-division multiple-access.
• Most modern systems use combinations: – TDMA/FDMA – CDMA/FDMA
Multiple-Access • Three ways to separate signals.
Code
– Frequency – Time – Code
Frequency
T
e im
FDMA – Frequency-division multiple-access. – Each user is assigned one frequency
Channel 1
30 kHz
2
3
4
guard band
frequency
FDMA
• Frequency-Division Multiple-Access • Examples:
Code
Ti
me
Frequency
– AMPS
TDMA • Time-division multiple-access • All users transmit at same frequency. • Each user transmits at a different time.
User 1 20 msec time slot
User 1 time
User 2 guard time
User 2 User 3
User 3
TDMA
• Time-Division MultipleAccess • Examples:
Code
Ti
me
Frequency
– USDC/IS-136
CDMA
• Code-Division MultipleAccess • Examples:
Code
Ti
me
Frequency
– IS-95 – Bluetooth
Wave Properties Lower frequencies, with longer wavelengths, are better suited to transmission over large distances, Higher frequencies, with shorter wavelengths, are better suited to transmission over small distances,
Fading dip point n位
Questions ?
History of wireless History of wireless communication archaic mobile communication • optical transmission (smoke/light
signals,..) • acoustic transmission (drums, alpine horns,...) Early telecommunications involved smoke, flags, drums, and other such methods to relay messages and information.
History of wireless History of wireless communication
The Wireless Telegraph
• Wireless (electrical) telegraph (Marconi). – 1899, Transmission across English channel. – 1901, 1st transatlantic communication. – 1907, Commercial ship-to-shore service.
Mobile Telephone Service (MTS) • MTS: – High power transmitter, at high elevation.
Same Frequecies
Washington
Call dropped
Baltimore
Improved Mobile Telephone Service (IMTS) • Improvements to MTS: – Full-duplex signaling, 1965. • No need for push-to-talk.
– Improved capacity • 120 kHz channels in 1946. • 60 kHz channels in 1950 (2x capacity). • 30 kHz channels in 1965 (4x capacity).
– Automatic trunking, 1965. • Allows more subscribers.
The Birth of Cellular •
Problem with IMTS: –
•
Not enough channels for the demand.
Solution: 1)
Release more bandwidth. •
1)
1974-1975, FCC released 80 MHz of bandwidth previously used by UHF television.
Break the coverage region into cells •
The cellular concept.
Questions ?